Phillips said the toddler's mother found her son in his car seat in the back of her vehicle and began shouting for help.

Neighbours rushed over and tried to resuscitate the toddler, according to the Fort Worth Star-Telegram. The child did not regain consciousness.

Jackson was taken to Cook Children's Hospital in Fort Worth where he was pronounced dead. Initial investigations found that he may have been in the vehicle for five hours.

The woman's family told NBC 5 that she had gone home to drop off her two children. The older child got out of the vehicle and the mother believed that the second child's father had taken him in from his car seat. She then drove off to visit a family member where she stayed for about five hours.

On the drive back home, she realised that her son was still in the back seat. He would have turned two years old on 6 May.

Forecasters said that temperatures in Burleston that afternoon were in the low 80s F (mid 20s C), KWTX reported.

"I think a lot of people aren't aware that even if it's 60, 70, 80 degrees outside it doesn't mean that the interior of a vehicle can't get well into 100 degrees," Sgt Jae Berg said.

Phillips said that no one has been arrested but the investigation was ongoing. Cause of death was still being assessed.

A GoFundMe page has been set up to help the family with funeral expenses, NBC Dallas-Fort Worth reported.